You have a FEL right?
That being the case, place your stand alone open center aux valve in series with the loader valve's power beyond outlet. I have posted several dozen times the correct method for plumbing an aux valve using this scenerio.
If you don't have a FEL it is slightly easier. You just connect your open center aux valve with power beyond in the exact same maner in which you would connect the loader valve using the tractor's front hydraulic block. A diagram of the block can be found in the loader manual which can be downloaded from the internet. Sorry, but I don't have the link handy right now. I'll post it later tonite if you need it.